1. Field of the Invention
The present invention relates to an optical parametric circuit used for wavelength conversion of optical signals, parametric amplification of optical signals, optical phase conjugation (spectral inversion) and for all-optical switching by utilizing the third-order optical parametric effect induced in a nonlinear optical medium, and to optical circuits using the same, for example, optical logic circuits, optical time-division multi/demultiplexers and/or optical sampling circuits.
2. Description of the Related Art
FIG. 53 shows a configuration of a conventional optical parametric circuit.
This figure illustrates a process of generating the third-order optical parametric effect by coupling a signal wave S (of carrier angular frequency .omega..sub.s) shown in FIG. 54A with pump waves P.sub.1, P.sub.2 (of carrier angular frequencies .omega..sub.P1, .omega..sub.P2) shown in FIG. 54B into an optical wavelength-division multiplexer 11 (refer to FIG. 54C) followed by a simultaneous inputting into a nonlinear optical medium 12 and propagating therefrom to induce the third-order optical parametric effect. The signal wave S is thus amplified (which is denoted by S'), and, through a four-wave mixing process, four-wave- mixing (shortened to FWM hereinbelow) wave F having a carrier angular frequency .omega..sub.f is generated (refer to FIG. 54D).
Here, the carrier angular frequencies .omega..sub.S, .omega..sub.P1, .omega..sub.P2, .omega..sub.f of the signal wave S, pump waves P.sub.1, P.sub.2 and FWM wave F, are governed by the law of conservation of energy as expressed in the following equation: EQU .omega..sub.S +.omega..sub.f =.omega..sub.P1 +.omega..sub.P2
The FWM wave F has a mirror symmetry with the spectrum of signal wave S with respect of the carrier angular frequency (.omega..sub.P1 +.omega..sub.P2)/2, and functions also as the optical phase conjugation wave for the signal wave S. In other words, this optical parametric circuit can function as a phase conjugation wave generation circuit. FIG. 55 shows a circuit configuration using a degenerate pump wave P (carrier angular frequency .omega..sub.P) and FIGS. 56A to 56D show the spectra of the optical waves corresponding to the spectra shown in FIGS. 54A to 54D. In the drawings, the signal wave S and FWM wave F are shown as right-angle triangular mirror images, for illustrative purposes, to indicate the fact that the FWM wave F is the phase conjugate wave of the signal wave S.
Such an optical parametric circuit can also serve as a wavelength conversion circuit to perform simultaneous wavelength conversion of each wavelength of wavelength-division multiplexed signals. For example, upon injection of an N number of signal waves S.sub.1 .about.S.sub.N (carrier angular frequency .omega..sub.S1 .about..omega..sub.SN), FWM waves .omega.F.sub.1 .about.F.sub.N are generated (carrier angular frequency .omega..sub.f1 .about..omega..sub.fN where .omega..sub.fj =.omega..sub.P1 +.omega..sub.P2 -.omega..sub.Sj for j=1.about.N) thus providing simultaneous wavelength conversion of each wave of the wavelength-division multiplexed signals. FIG. 57 shows a circuit configuration based on a degenerate pump wave P (carrier angular frequency .omega..sub.P). FIGS. 58A to 58D show the spectra corresponding to the spectra shown in FIGS. 56A to 56D. In these figures, filled and unfilled triangles are used for showing the correspondence between signal waves S.sub.1 .about.S.sub.N and FWM waves F.sub.1 .about.F.sub.N.
An example of application of the parametric circuit as an optical amplification circuit is shown in FIG. 59. Propagation patterns of each wave to the output port of the nonlinear optical medium 12 are the same as those presented in FIGS. 54A.about.54D. The amplified signal S' is shown in FIG. 54D.
Using the configuration shown in FIGS. 53, 55 and 57, signal wave S (S.sub.1 .about.S.sub.N), pump waves (P.sub.1, P.sub.2 and P) and FWM waves (F.sub.1 .about.F.sub.N) are all output colinearly from the nonlinear optical medium 12, therefore, to obtain only the FWM waves F (F.sub.1 .about.F.sub.N), it is necessary to employ a wavelength filter 13 which passes only those waves having carrier angular frequencies .omega..sub.f (.omega..sub.f1 .about..omega..sub.fN). The output spectra from the wavelength filter 13 are shown in FIGS. 54E, 56E and 58E. By using the configuration as an optical amplification circuit of FIG. 59, it is necessary to use a wavelength filter 29 which passes only the amplified signal wave S' of carrier angular frequency .omega..sub.S. The output spectrum from the wavelength filter 29 is shown in FIG. 60.
When the optical parametric circuit is to be used as an FWM wave generator, it is necessary to pack the carrier angular frequencies .omega..sub.S, .omega..sub.P1 and .omega..sub.P2 to increase the conversion gain (expressed as FWM wave intensity/signal wave intensity) of signal wave S to FWM wave F, as well as increase the pump wave intensity. Similarly, when the optical parametric circuit is to be used as an optical parametric amplifier, it is necessary to pack the carrier angular frequencies, .omega..sub.S, .omega..sub.P1, .omega..sub.P2, and increase the pump wave intensity to increase the amplification gain of the signal wave.
Therefore, it is necessary for the wavelength filter 13 to possess a capability to suppress the pump wave intensity, which is higher relative to the FWM wave intensity, but pass the FWM waves having carrier angular frequencies which are closely packed with respect to the pump waves. However, it is difficult to sufficiently suppress the pump wave using only one wavelength filter, and therefore, it is general practice to employ a multi-stage optical filter. Therefore, the FWM wave or the signal wave suffers loss of the power and limitation of the bandwidth, and the circuit arrangement has been complicated. In the case of using the optical parametric circuit as the optical amplification circuit, there have been similar problems.
Also, in the conventional optical parametric circuits, when a phase conjugate signal (i.e. FWM wave F) is generated upon injection of an signal wave S, the carrier angular frequency is shifted from .omega..sub.S to .omega..sub.f (=.omega..sub.P1 +.omega..sub.P2 -.omega..sub.S), using the examples of FIGS. 56A and 56E. This presents a problem that when using the optical parametric circuit as a phase conjugation circuit in an optical fiber transmission system, the carrier angular frequency of transmitted optical signals is changed by passing through the phase conjugation circuit.
Furthermore, when the optical parametric circuit is to be used as a simultaneous wavelength conversion circuit for wavelength-division multiplexed signals, if the carrier angular frequencies .omega..sub.Sm, .omega..sub.Sn of the two signal waves S.sub.m, S.sub.n and the carrier angular frequencies .omega..sub.P1, .omega..sub.P2 of the pump waves are related in such a way to satisfy the equation: EQU .omega..sub.Sm +.omega..sub.Sn =.omega..sub.P1 +.omega..sub.P2
then the carrier angular frequencies of the FWM waves F.sub.m and F.sub.n are set to .omega..sub.Sn and .omega..sub.Sm, meaning that the effect of interchanging the carrier angular frequencies of the signal waves would be obtained. However, in the conventional optical parametric circuits, the carrier angular frequencies of the FWM wave F.sub.m becomes equal to that of the signal wave S.sub.n, and similarly, the carrier angular frequencies of the FWM wave F.sub.n becomes equal to that of the signal wave S.sub.m. Therefore, it is not possible to separate the FWM waves F.sub.m and F.sub.n from the signal waves S.sub.n and S.sub.m using any wavelength filtering devices.